The present invention relates generally to electrochemical synthesis methods for producing high purity hydride gases for semiconductor fabrication and doping. The invention relates more particularly to the electrochemical synthesis and production of Group IV and V volatile hydrides such as phosphine, arsine, stibine, and germane.
As further background, high purity gases are required for semiconductor fabrication and doping. Often these gases are extremely toxic and hazardous. Hence, centralized production, transportation and storage of these materials presents a hazard to those working with them. On-site electrochemical synthesis of these gases provides an alternative means to provide such gases to the semiconductor industry in a safe manner. The process described below allows the gas to be generated as needed thereby minimizing the amount of gas present prior to use in a semiconductor fabrication reactor. This provides a substantial advantage over the use of compressed gas in cylinders. Commercial compressed gas cylinders store gas at several thousand pounds per square inch pressure and contain one to ten pounds of gas. Hence, gas cylinders present a major chemical release hazard. On-site electrochemical generation of the gas eliminates this hazard.
The following references disclose processes for producing these gases by chemical methods. Cotton and Wilkinson, "Advanced Inorganic Chemistry", Wiley Interscience, Fourth Ed. (1980) and Brauer, "Preparative Inorganic Chemistry", Academic Press (1963) teach that the Group IV and V hydrides can be produced by chemical reduction of electropositive compounds of the desired product gas element with acids or the reduction of the halides with LiAlH.sub.4 or NaBH.sub.4. For example: EQU Na.sub.3 P+3 H.sub.2 O.fwdarw.PH.sub.3 +NaOH EQU Mg.sub.3 Sb.sub.2 +6 HCl.fwdarw.2 SbH.sub.3 +3 MgCl.sub.2 EQU Na.sub.3 As+3 NH.sub.4 Br.fwdarw.AsH.sub.3 +3 NaBr+3 NH.sub.3 EQU Mg.sub.2 Ge+4 NH.sub.4 Br.fwdarw.GeH.sub.4 +2 MgBr.sub.2 +4 NH.sub.3 EQU GeCl.sub.4 +LiAlH.sub.4 .fwdarw.GeH.sub.4 +LiCl+AlCl.sub.3
These gases can also be prepared by the electrochemical reductions: EQU Sb+3 H.sub.2 O+3e.fwdarw.SbH.sub.3 +3 OH-- EQU As+3 H.sub.2 O+3e.fwdarw.AsH.sub.3 +3 OH-- EQU Ge+3 H.sub.2 O+3e.fwdarw.GeH.sub.3 +3 OH-- EQU P+3 H.sub.2 O+3e.fwdarw.PH.sub.3 +3 OH--
In addition, dissolved ionic precursors can be used such as: EQU H.sub.2 PO.sub.2 --+5 H++4e.fwdarw.PH.sub.3 +2 H.sub.2 O
Salzberg, J. Electrochem. Soc., 101:528 (1964) discloses the electrochemical formation of stibine at an antimony cathode. Lloyd, Trans. Faraday Soc., 26:15 (1930) and Salzberg, J. Electrochem. Soc., 107:348 (1960) disclose the preparation of high purity arsine at an arsenic cathode. Spasic, Glas. Hem. Drus. Beograd., 28:205 (1963) discloses the electrochemical production of germanium hydride.
E. W. Haycock and P. R. Rhodes (U.S. Pat. No. 3,404,076) disclose a method for the electrolytic preparation of volatile hydrides. Gordon and Miller (U.S. Pat. Nos. 3,109,785 and 3,109,795), Miller and Steingart (U.S. Pat. No. 3,262,871) and Miller (U.S. Pat. No. 3,337,443) disclose electrolytic methods for the production of phosphine.
Porter, in U.S. Pat. No. 4,178,224, discloses an electrochemical method for the synthesis of arsine gas. His method utilizes a dissolved arsenic salt with an oxygen evolving anode. With this method, the arsine concentration was limited to less than 25%. Another limitation of Porter's method was the need to balance pressures and liquid levels in the divided anode and cathode sections of the electrochemical cell. This requires an inert gas supply to the cell.
W. M. Ayers, in U.S. Pat. No. 5,158,656, describes an electrochemical apparatus and method for supplying volatile hydrides at the proper pressure for introduction into a chemical vapor deposition reactor.
While efforts have continued to provide effective means for producing and delivering hydride gases, needs still exist related to the quality and consistency of delivered product streams including hydride gases. The present invention addresses these needs.